Generally, gases generated from vehicles are classified into three types according to gas sources, that is, exhaust gas from an exhaust pipe, blowby gas generated from the crank case of an engine, and evaporation gas from a fuel tank or a vaporizer.
Of the gases from vehicles, the exhaust gas is discharged to the atmosphere from the exhaust pipe after fuel is burnt in an engine cylinder, and has a complicated composition consisting of noxious gas components and harmless gas components. The harmless gas components include H2O and CO2, whereas the noxious gas components include CO, hydrocarbon (HC), NOx, lead compounds and carbon particles. However, the main noxious components that may cause environmental pollution are the NOx, CO and HC. The volume of the discharged noxious gas components varies depending on various parameters including engine dimension, engine running condition, etc.
The blowby gas is discharged from a gap between the piston and the cylinder to the crank case, and is also called crankcase emission. Such blowby gas is a mixture of gases which is composed of 70–95% unburnt gaseous HC, burnt gas and partially oxidized gas. The corrosion of the engine and the degradation of oil are undesirably caused when the blowby gas is left in the crank case. In order to overcome this problem, the crank case is conventionally configured to be sufficiently ventilated and discharge the blowby gas to the atmosphere. However, when using this configuration, the content of the noxious HC in the discharged blowby gas is undesirably high, and thus it is necessary to discharge the blowby gas to the atmosphere after reburning the blowby gas.
When the exhaust gas is directly discharged to the atmosphere through an exhaust pipe, the volume of discharged sooty smoke and noxious gas is relatively small when the engine is driven at low speeds under 1,500 RPM, whereas the volume of the discharged exhaust gas is increased at high speeds above 1,500 RPM, so the volume of the discharged sooty smoke and noxious gas included in the exhaust gas is undesirably increased, with the sooty smoke and noxious gas being the principal cause of air pollution.
The exhaust gas discharged to the atmosphere is problematic in that a variety of noxious components of the exhaust gas including carbon monoxide, hydrocarbon, sulfur, nitrous oxide, aldehyde and suspended particles, are discharged to the atmosphere. In order to prevent such noxious components from being produced in the internal combustion engine, the combustion rate of fuel in the internal combustion engine must be increased, and an exhaust gas control device is required to be installed, in addition to a requirement to treat unburnt materials, and prevent evaporation of fuel.
Since it is impossible to completely burn the fuel in the engine cylinder of a vehicle, a variety of noxious liquid components contained in the exhaust gas produced from the internal combustion engine are discharged to the atmosphere along with other components of the exhaust gas.
Such noxious liquid components discharged with other components of the exhaust gas are converted to sooty smoke or carbon while being attached to the inner wall of the exhaust pipe due to its having a lower temperature than the combustion chamber. A diesel engine has a problem that it does not use a catalytic converter at the exhaust pipe, so the non-purified exhaust gas is directly discharged to the atmosphere and causes air pollution.